[Feature Request]: A turn's prompt= should extend the agent's prompt, not silently replace it #3324

Is your feature request related to a problem? Please describe.

Agent._prepare_turn seeds the turn's prompt from the agent's system prompt and dynamic prompts only when the caller passes none (ag2/agent.py:1264: if not context.prompt: ...). A caller that passes prompt= to run() / ask() to add per-turn context therefore silently replaces everything the agent was built with — including the text plugins contribute, such as SkillPlugin's <available_skills> catalog — and dynamic prompts never run.

We hit this in ag2-assistant: the gateway passed per-turn guidance as prompt=, and for months no skill description reached the model; skills were only discovered when the user named them. The fix was to prepend agent.system_prompt by hand, which still skips dynamic prompts.

Describe the solution you'd like

A way to extend the agent's prompt per turn rather than replace it, e.g.:

• run(..., extra_prompt=[...]) appended after the agent's system + dynamic prompts, or
• prompt= semantics changed to extend by default, with an explicit replace_prompt=True for today's behaviour.

Either way dynamic prompts (and plugin prompts) should still run for a turn that adds its own text. At minimum, document the replacing behaviour on run() / ask().

Additional context

• ag2/agent.py — _prepare_turn
• Related: [Feature Request]: SkillPlugin renders its skills catalog per turn #3325 (SkillPlugin catalog per turn).
• Motivation: feat(a2ui): Cards become files, drawn from a shared vocabulary ag2-assistant#117 (the fix on the app side), Agent capabilities apply on the next message, not on the next reload ag2-assistant#121.